The subject of the present invention is a hydrocarbon conversion process for reforming of naphtha charge stocks containing low levels of sulfur that utilizes a novel trimetallic catalytic composite which has exceptional activity and resistance to deactivation when employed in a hydrocarbon conversion process that requires a catalyst having both a hydrogenation-dehydrogenation function and a cracking function. More precisely, the present invention involves processing a naphtha charge stock containing less than 300 wt. ppb sulfur over a novel dual-functional trimetallic catalytic composite which, quite surprisingly, enables substantial improvements in hydrocarbon conversion processes, specifically, improved activity, selectivity, and stability characteristics.
Catalyst composites having a hydrogenation-dehydrogenation function and a cracking function are widely used today as catalysts in many industries such as the petroleum and petrochemical industry to accelerate a wide spectrum of hydrocarbon conversion reactions. Generally, the cracking function is thought to be associated with an acid-acting material of the porous, adsorptive, refractory oxide type which is typically utilized as the support or carrier for a heavy metal component such as the metals or compounds of metals of Groups V through VIII of the Periodic Table to which are generally attributed the hydrogenation-dehydrogenation function.
These catalytic composites are used to accelerate a wide variety of hydrocarbon conversion reactions such as hydrocracking, isomerization, dehydrogenation, hydrogenation, desulfurization, cyclization, alkylation, polymerization, cracking, hydroisomerization, etc. In many cases, the commerical applications of these catalysts are in processes where more than one of these reactions are proceeding simultaneously. An example of this type of process is reforming wherein a hydrocarbon feed stream containing paraffins and naphthenes is subjected to conditions which promote dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization of paraffins and naphthenes, hydrocracking of naphthenes and paraffins and the like reactions to produce an octane-rich or aromatic-rich product stream. Another example is a hydrocracking process wherein catalysts of this type are utilized to effect selective hydrogenation and cracking of high molecular weight unsaturated materials, selective hydrocracking of high molecular weight materials, and other like reactions, to produce a generally lower boiling, more valuable output stream. Yet another example is an isomerization process wherein a hydrocarbon fraction which is relatively rich in straight-chain paraffin compounds is contacted with a dual-function catalyst to produce an output stream rich in isoparaffin compounds.
Regardless of the reaction involved or the particular process involved, it is of critical importance that the dual-function catalyst exhibit not only the capability to initially perform its specified functions, but also that it has the capability to perform them satisfactorily for prolonged periods of time. The analytical terms used in the art to measure how well a particular catalyst performs its intended functions in a particular hydrocarbon reaction environment are activity, selectivity and stability. And for purposes of discussion here, these terms are conveniently defined for a given charge stock as follows: (1) activity is a measure of the catalyst's ability to convert hydrocarbon reactants into products at a specified severity level where severity level means the specific reaction conditions used--that is, the temperature, pressure, contact time, and presence of diluents such as H.sub.2 ; (2) selectivity refers to the amount of desired product or products obtained relative to the amount of reactants charged or converted; (3) stability refers to the rate of change with time of the activity and selectivity parameters--obviously, the smaller rate implying the more stable catalyst. In a reforming process, for example, activity commonly refers to the amount of conversion that takes place for a given charge stock at a specified severity level and is typically measured by octane number of the C.sub.5.sup.+ product stream, selectivity refers to the amount of C.sub.5.sup.+ yield that is obtained at a particular activity level; and stability is typically equated to the rate of change with time of activity, as measured by octane number of C.sub.5.sup.+ product, and of selectivity, as measured by C.sub.5.sup.+ yield. Actually, the last statement is not strictly correct because generally a continuous reforming process is run to produce a constant octane C.sub.5.sup.+ product with severity level being continuously adjusted to attain this result; and, furthermore, the severity level is for this process usually varied by adjusting the conversion temperature in the reaction zone so that, in point of fact, the rate of change of activity finds response in the rate of change of conversion temperature and changes in this last parameter are customarily taken as indicative of activity stability.
As is well known to those skilled in the art, the principal cause of observed deactivation or instability of a dual-function catalyst when it is used in a hydrocarbon conversion reaction is associated with the fact that coke forms on the surface of the catalyst during the course of the reaction. More specifically in these hydrocarbon conversion processes, the conditions utilized typically result in the formation of heavy, high molecular weight, black, solid or semi-solid, carbonaceous material which coats the surface of the catalyst and reduces its activity by shielding its active sites from the reactants.
Equally detrimental to the stability of a hydrocarbon conversion process is contamination of the catalyst by impurities in the feed being processed. Specifically, water, sulfur, nitrogen, and certain metallic elements, such as arsenic or lead, are well known catalyst poisons and are to be especially avoided. Of these contaminants, water and sulfur are the most common and if not held to low levels in the feed, they can cause rapid catalyst deactivation. Such deactivation results in more frequent catalyst regenerations which in turn increases down time and reduces the expected useful life of the catalyst.
In other words, the performance of this dual-function catalyst is sensitive to both the presence of carbonaceous deposits on the surface of the catalyst and contaminants in the charge stock. Accordingly, the major problem facing workers in this area of the art is removal of feed contaminants and the development of more active and selective catalytic composites that are not as sensitive to the presence of these carbonaceous materials and/or have the capability to suppress the rate of the formation of these carbonaceous materials on the catalyst. The sensitivity to contaminants and formation of carbonaceous materials is amplified as practitioners of the art reduce pressure and increase the severity of processing units in an attempt to extract the maximum octane-barrels from a given feedstock. Viewed in terms of performance parameters, the problem is to develop a process utilizing a dual-function catalyst having superior activity, selectivity and stability while operating at pressures less than 963 kPa.